Refrigeration systems are designed to produce a cooling effect. These systems are typically used for the cool storage of food and for air conditioning.
The basic principle of refrigeration is cause a substance to undergo a physical change from liquid to vapor in order to obtain the cooling effect.
The substance is commonly referred to as the refrigerant. The refrigerant is generally contained in a closed, pressurized system having two major portions, the condenser and the evaporator. During a refrigeration cycle, the refrigerant in liquid form is supplied from the condenser to the evaporator where it is caused to change to its vapor phase. The heat to be removed is absorbed by the evaporator to evaporate the liquid refrigerant. The absorbed heat is then expelled in the condensor by condensing the refrigerant vapors into liquid form. The cycle is then continuously repeated.
The most commonly used refrigerants are known as chlorofluorocarbons (CFCs). These CFCs are generally fully halogenated, meaning that the hydrogen atoms of the hydrocarbon have been replaced with a halogen such as chlorine or fluorine.
CFCs are now believed to cause the destruction of the ozone layer of the Earth. The ozone layer is located in the upper layer of the atmosphere, known as the stratosphere. Ozone (O.sub.3) is formed by an interaction of molecular oxygen with the ultraviolet radiation (UV) of the sun. The presence of chlorine in the stratosphere is believed to cause the breakdown of ozone. The chlorine contained in the CFCs is believed to be the major source of chlorine in the stratosphere. It is believed that unless the amount of chlorine in the stratosphere is reduced, the stratosphere will not be capable of generating sufficient ozone and the harmful UV radiation of the sun will be let through to the surface of the Earth. A recent international agreement will limit the production of most of these CFCs in order to reduce the depletion of the ozone layer.
This agreement has caused intense concern in the refrigeration and air conditioning industry about the availability and effectiveness of substitute refrigerants. While it is believed that the development of substitute refrigerants that do not destroy the ozone will occur, there is some hesitation that the qualities or properties of the current CFC type refrigerants might be compromised.
To be useful as a refrigerant, any new refrigerant must have the following characteristics:
Chemical Stability--the refrigerant should not decompose or react with other components or contaminants in the refrigeration system, preferably, the refrigerant is completely stable in the system, yet should be capable of decomposing in the atmosphere (by UV radiation or chemical reaction) before it reaches the ozone layer.
Safety--the selected refrigerant should have a low order of toxicity properties and should be non flammable.
Thermodynamic Properties--the selected refrigerant should have suitable critical or boiling point temperatures and heat capacity in its vapor stage in order to be useful as a refrigerant. Generally, a suitable refrigerant should have a freezing point of less than -40.degree. F. (-40.degree. C.), a critical temperature greater than 122.degree. F. (50.degree. C.), a vapor pressure at 176.degree. F. less than 735 psia (5 MPa) and a latent heat times vapor density greater than 27.8 Btu/ft.sup.3 (1.0 kJ/l).
Other desirable parameters for the substitute refrigerants include high oil solubility, the ability to blend with other refrigerants, high vapor dielectric strength, compatibility with the materials of the refrigeration system, especially copper, easy leak detection and low cost.
Current attempts to provide new refrigerants have focused upon other CFCs which are not fully halogenated and therefore are likely to break down in the lower atmosphere. Various blends of such CFCs have been proposed. Additionally, in article entitled "Quest For Alternatives", by McLinden and Didion in ASHRAE Journal, Dec., 1987, the authors also suggest that the search for CFC alternatives focus on these non fully halogenated CFCs rather than non-chlorofluorocarbon compounds.